1. Field of the Invention
The present invention relates to dynamic microphone handsets and amplifier units in telephone circuitry and particularly to ground fault isolation circuitry associated therewith.
2. Prior Art
Microphone amplifiers for the microphones used in a telephone handset are quite well known and varied. In many cases, a carbon microphone is used in the handset because of the low cost and a certain level of background noise suppression that is inherent in this type of microphone. Dynamic microphones, however, are replacing carbon granule microphones because of greater audio fidelity and lower internal noise.
The replacement of a carbon microphone with a dynamic microphone involves more than a simple microphone device swap because of the need to ensure that the proper circuitry, including the signal amplifier is included.
In addition to the need to interface to the existing signal amplifier in the event of a retrofit change in microphone, pay telephone stations are vulnerable to a type of fraud known as "pinning" involving the introduction of a ground fault in the handset circuitry. Accordingly, there is a need to provide an amplifier for a dynamic microphone, either installed as original equipment or as a replacement, that eliminates ground-fault initiated fraud. Also, an electric shock hazard can be present if continued hard use results in chafing and breakdown of the insulation on the wires routed inside the usual armored cable that connects the handset to the pay station housing.
The present invention includes the placement of the dynamic microphone amplifier in the pay station housing rather than in the handset. Furthermore, an isolation transformer is used to prevent ground faults in the transmitter portion of the handset from being introduced in a manner that allows fraudulent use of the phone. Quite importantly, the dynamic microphone amplifier is mounted on a printed circuit board having a second isolation transformer for the receiver circuitry and speaker element in the handset. The four leads to the handset are wired into a 4-pin connector plug that is physically mounted within the pay station housing. Thus, the present invention provides distinct advantages over the usual dynamic microphone amplifiers, such as that disclosed in U.S. Pat. Ser. No. 4,378,467, which cannot be used to prevent fraud. The present invention provides for ease of installation and ground fault isolation in addition to amplification of the dynamic microphone signal.